The present invention concerns strengthening of structures and more specifically strengthening of structures according, to the additional prestressing method.
Additional prestressing is a known method for strengthening or repairing concrete or masonry structures and has formed the subject of standard NF P95-104 (“Repair and strengthening of concrete and masonry structures”, published in December 1992 by the French standards association (AFNOR)).
It involves generating prestressing forces by putting reinforcing members into tension, for example prestressing cables, external to the structure to be strengthened. The reinforcing members transmit forces to the structure through anchorages bearing on spirally reinforced concrete or metal parts, called bosses, offset from structural components such as existing bracings, beams or walls.
As specified in section 5.2 of standard NF P 95-104, these anchorages are generally fixed by pinning using prestressing bars.
Concrete bosses are voluminous units, which can either be cast in place or precast. However, boss installation proves delicate in either case, especially because of the difficulty in accessing structural components on which bosses must bear. Openings or windows must sometimes be created in the structure to allow bosses to be installed.
Metal bosses are shop-fabricated for adaptation to each structure, which ensures their superior fabrication quality compared with concrete bosses. To allow such bosses to have a surface with a sufficiently high friction coefficient bearing on a concrete structural component, this bearing surface must be serrated by machining it or by welding steel wires to it, leading to high construction cost.
According to the method recommended by standard NF P 95-104, reinforcing members pass through the bosses in which they are anchored. Such an anchorage is always made inside the boss on the side opposite to the reinforcing member regular section. It thereby compresses the boss when the reinforcing members are tensioned.
FIG. 1 shows an example of such an anchorage. In the embodiment illustrated in this figure, a boss 1 bears on a bracing 3 of the structure to be strengthened. A prestressing member 2 passes through boss 1 for anchorage therein, at the end opposite the regular section of the prestressing member 2.
Moreover, bosses foreseen by the standard are positioned at the ends of the structure to be strengthened, such that the additional prestress is applied over the longest possible distance. They are therefore often placed near to obstructions, such as structural walls, columns or bracings. This arrangement makes it difficult to place the reinforcing-member tensioning jack.
For this reason, it is generally necessary to displace the boss away from the obstruction to ensure enough clearance to position the jack, which effectively limits prestressing to only a subsidiary part of the structure. Alternatively, the boss may be placed near the obstruction on condition, however, that an opening is made in this obstruction to allow the jack to be positioned. Applying the resulting prestress is better than in the former case, but this solution requires concrete break-out or drilling work to be performed.
One object of the present invention is to overcome the drawbacks of the current methods recalled above by allowing prestress to be applied to a major section of the structural component to be strengthened, whilst avoiding implementation of work involving breaking out part of the structure for strengthening purposes.
Another object of the invention is to have an anchorage well suited to the structure to be strengthened, without requiring additional adaptation operations such as machining of an anchorage block surface bearing on a member of the structure to be strengthened.